EPT+ Show Spoiler [Blizzcon presentation] +
If anyone cares, that's Blizzard's presentation of the unit. There are three factors to keep into consideration.
- First is Blizzard's stated reasoning for creating the unit. Roughly, protoss lacks harassing units, so they want to add one.
- Second is the unit concept they created to address the problem described in point one. To reiterate, the oracle is a flying spellcaster that can deal economic damage, but has no combat abilities of any kind, because Blizzard does not want to add to protoss' army strength.
- Third is the actual unit as it exists in the HotS version available at Blizzcon. I won't describe this here, as the video shows this fairly well.
These three factors are closely related. You can see them as 1. problem 2. solution 3. implementation. Any one of them is influenced by the other and can independently sabotage Blizzard's effort: the problem description influences the unit design, unit design influences implementation. As the oracle is not the only change to the game, new problems could manifest themselves and make the entire problem description outdated. There are a lot of unforseen factors in making such wide-ranging changes, as it is still up to the player-base to adopt a new unit and find uses for it in competitive play. Still in Wings of Liberty there are units whose popularity fades in and out and you're never sure whether it's a design issue or just some expected variance in strategic trends. If we can't be sure in this case, what hope do we to make the right judgement with a completely new unit in a heavily changed game? Because of these circumstances, it's also possible to create a tremendously flawed design that just happens to work out in practice. It's prudent to stay cautious and let units undergo rigorous testing before commenting on viability.
The goal of this post is therefore not to comment on whether the unit will work or not, or even what a good unit would look like*, things that usually are impossible to tell. Rather, the point is to improve the process at which Blizzard arrives at their final unit implementations. Design is somewhat of an arcane art, but it's not completely based on lucky guesses. Compare it to the smoke and mirrors metaphor. Smoke can prevent you from discerning more than the outline of an object, mirrors are traps that can make you think you're seeing something different than you are. You want to smash the mirrors and blow away the smoke to provide clarity.
*+ Show Spoiler +
I want terran to get a mechanical mole that can burrow to undermine buildings and cause their collapse. 
So let's try a more abstract version of the oracle to see if any problems jump out. As seen in Blizzcon, it's a very fast, flying and fragile unit. Artosis said it was at least as fast as a phoenix, the fastest flying unit in the game. I would contend that because it has an energy based form of harass, you can simply fly it in every time the timer comes up. With instant-cast spells and at least a minimum amount of shields it simply can't be scared off with conventional means. You're not going to be able to prevent it from casting a spell unless you devote quite a lot to stopping it.
Blizzard could try a version without shields to always ensure at least some damage done, they could make its casting range lower to provoke it into getting into danger. It's possible to balance the unit around both its movability and energy use. Note that the same goes for the phoenix, but at least one phoenix becomes vulnerable while using its lift ability, and as you're able to use them in combat it's not likely they can survive the entire game. The oracle has no such problems, it can more easily stay alive and has less reason to risk itself.
Because of the difficulty of multi-tasking units it might be so the oracle is less invulnerable than thought. It's easy to slip up after all. At the same time, you can time out when you want to use them and maximize the chance they stay alive.
Decision making with the oracle does exist. You have to decide when its phase out ability can be useful, but that won't be the case most of the time. Usually you just cast entomb and you leave, it's a bit binary. For the opposing player the question is how many units to leave behind to kill off the mineralfields.
The oracle seems like it can have potential for some pushes (mass air in PvP/Z and then phase out the cyber core/hydra den to prevent anti-air production), in some compositions (void ray/phoenix builds with an oracle to shut down static defense). It can be used for mineral harass, for scouting. There are plenty of reasons to think it can be balanced properly and be a useful unit in a wide variety of circumstances, but the question remains whether this form of energy based flying harass is a good idea.
You can tweak some of the rules for the unit slightly. For instance, what if it was made slower? Would some of the perceived one-dimensionality of it go away then? Well, then it'd die to blink stalkers, marines, mutalisks. It wouldn't need energy any more, as it most likely won't be given the time to regenerate it. Unlike something like a warp prism or medivac, it has no way to threaten any of those units, outside of having some back-up. But the oracle's abilities pale in comparison to the damage a void ray if left unattended can do. It's a stronger threat and will have to be dealt with more harshly, and in the battle of such powers the poor oracle will mostly just be collatoral damage.
What if we looked not at the oracle, but tried to change other units to work well with the new protoss unit? I don't think it's wise to go into this as it would require meddling with all the unit relationships for harass, so that's a lot of changes to the game. Blizzard could do it, but it'd be quite a project and really something that's outside the scope of this post.
I think Blizzard wants to go with a flying unit because it simply fits better with the race. They can have it replace the mothership, while a ground-based harass unit would almost have to replace the colossus. Then you need something that can be used by a warp prism, yet doesn't suddenly become too powerful with warp-in available. I don't want to speculate further on this though, I'm sure they have their reasons. I also don't want to speculate much about why protoss needs harassment units. Many of the top protoss players have been using blink stalkers and warp prisms more, but death balls are still the core of the protoss army. I guess it is easy to play in a way that is low on multi-tasking and harass and that Blizzard wants to have some tools in place to promote different styles.
I guess I mostly talked about the implementation part. The other two factors that I mentioned are possibly more interesting and worthy of discussion though, as after all we can all judge for ourselves whether we agree with their analysis of protoss.
I have a personal suggestion for the oracle - which I won't claim is great or anything, but it seemed cool to me - and it would be to make them a constant threat in such a way they're forced to risk themselves. I thought they could replace the entomb ability with a regular attack that does something vaguely equivalent. So you're encouraged to constantly fly over worker lines, exposing yourself to enemy fire more often than if you just casted a spell.
Anyway, feel free to discuss what you think of the unit. But don't forget that while it's okay to be critical, nobody knows how the unit will work, so we're still mostly just guessing.
